I dont know why i should not keep my coins in exchanger if i am carefully then i think that nobody cant stole my money from my wallet so i keep my money in exchanger really long time and it is still there in exchanger.
Such things does not happen everyday and not always with one exchange, today it is bitfinex tomorrow it can be any other, so for safety someone should just keep his funds in the wallet and just keep the funds in exchange which he is gonna use for trading.